Clot retraction time
Quick answer: Clot retraction time checks how well a blood clot shrinks and firms up over time, which depends mainly on platelets. It is an older test sometimes used when a platelet problem is suspected.
- SampleBlood
- TakesAbout 5 minutes for the blood draw
- Before the testNo special preparation is usually needed; tell staff about any blood thinners you take.
How the test is done
- 1Blood is drawn from a vein in your arm into a plain glass tube
- 2The tube is kept at body temperature in the lab
- 3Staff check how much the clot shrinks and pulls away from the tube
- 4The result is read after a set time
What your results can mean
Reading your report
Results are usually reported as normal or poor retraction; your doctor will read them with your platelet count.
If the result is outside the usual range
Poor clot retraction can be seen with a low platelet count or with platelets that do not work well, such as in Glanzmann thrombasthenia. Normal retraction suggests the platelets are doing their part in firming up the clot.
Who usually has this test
People with easy bruising or bleeding, usually alongside a platelet count and other clotting tests.
Risks and aftercare
You may have a small bruise; press firmly on the site, especially if you bruise easily.
Names and options
Also called: CRT, clot retraction test
Listed by the hospital as
- CLOT RETRACTION TIME
Questions about this test
Is clot retraction time still used?
It is an older, simple test that many labs have replaced with specific platelet function tests. It can still give a quick clue when a platelet problem is suspected.
Why does a clot shrink?
Platelets inside a clot contain proteins that contract, pulling the clot tighter. If platelets are too few or do not work properly, the clot stays loose.
Can medicines affect the result?
Some medicines, such as aspirin, can affect how platelets work. Tell your doctor about all your medicines, but do not stop any without their advice.
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This page explains each test in general terms so you know what to expect. It is not medical advice: your doctor decides which tests you need and what your results mean. Reference ranges differ between laboratories. Updated 4 November 2026.